Early initiation of intensive intervention may lead to improved outcomes for children with autism spectrum disorders (ASD);yet there are currently no reliable techniques for diagnosing ASD before the age of two years. This research is designed to identify variations in frequencies of occurrence, delayed onset, and/or atypicalities in vocal, motor, and communicative skills that may index later ASD diagnosis. The approach taken involves describing the development of these skills in infant siblings of older children who have been diagnosed with Autism. Infant Siblings are known to be at elevated risk for ASD and language delay. The goal will be to distinguish prospectively between those Infant Siblings eventually diagnosed with ASD, those eventually diagnosed with Language Delay but not ASD, and those with no apparent atypicalities. One hundred fifty Infant Siblings will be observed longitudinally from ages 5 to 14 months, with follow-up at 18, 24, and 36 months. Audio- and videotaped observations designed to sample behavior in contexts that are consistent across participants, standardized instruments, and parental reports will be employed to study vocalizations, rhythmic limb movements, vocal-motor coordinations, postural and locomotor abilities, gross, fine, and oral-motor skills, words, and gestures. Diagnostic outcomes will be assessed at 36 months with the Autism Diagnostic Observation Schedule, Autism Diagnostic Inventory-Revised, and clinical judgment using DSM-IV criteria. Data from all three subgroups of Infant Siblings will also be compared to parallel data from infants with no known risk for ASD currently being followed as part of separate NAAR and NIH-funded studies. Analyses will focus on identifying behavioral markers in infancy that predict a later diagnosis of ASD, assessing their positive predictive value, sensitivity, and specificity, and on evaluating a set of hypotheses concerning variation in patterns of production and developmental trajectories of infant vocal, motor, and communicative behaviors that may differentiate groups and predict an eventual ASD or Language Delay diagnosis. We currently lack reliable methods for diagnosing autism spectrum disorders (ASD) in children younger than 2 years of age. To identify possible predictors of later ASD diagnosis, this study will focus on the development of vocal, motor, and communicative skills in infants at particular risk for ASD and relate atypicalities in infant development to diagnostic status at 3 years of age.